The present invention relates to a hybrid injection unit, and to an injection molding machine provided with a hybrid injection unit.
Injection molding machine typically include a clamping unit and a plasticizing or injection unit. The clamping unit normally includes at least two platens for attachment of the molding members or half-molds. As the half-molds are closed, a mold cavity is formed which receives plastic melt injected by the plasticizing unit. After the melt has cured, the clamping unit is opened and the molded article is removed from the cavity. The plasticizing unit transforms the initial material, such a plastic pellets, into a melt usually by applying energy such as shear energy. The melt accumulates in the screw end chamber of a cylinder and screw assembly. Plasticizing takes place as the plasticizing screw rotates in the plasticizing cylinder. Injection of plastic melt into the cavity is realized by moving the plasticizing screw forward in axial direction so that the melt in the screw end chamber is pushed into the cavity of the injection mold that forms part of the clamping unit.
The afore-described operations require the provision of several drives. The clamping unit has to be closed and opened. Moreover, the injection process requires application of a clamping pressure. The plasticizing screw for carrying out the plastification is caused to rotate by a suitable drive. In addition, the axial displacement of the plasticizing screw is realized by a drive. Many approaches have been proposed to date for implementing the various drives. One approach involves the use of a hydraulic drive for closing the clamping unit as well as applying the clamping pressure. Also the plasticizing screw of the plasticizing unit can be rotated and axially displaced by hydraulic means. More recently, the use of electric drives has been proposed for operating the clamping unit (oftentimes coupled with a toggle mechanism) as well as the plasticizing unit for driving the plasticizing screw. Electric drives are precise, easy to control and react and respond fairly quickly. Another approach involves combinations of hydraulic and electric drives for the plasticizing screw.
German Pat. No. DE 43 44 335 describes the use of two electric motors for driving the plasticizing screw. Both electric motors are constructed with two fixed stator and rotor assemblies. One of the electric motors is hereby intended to axially move the plasticizing screw via a spindle and nut combination, whereas the other electric motor is provided to rotate the plasticizing screw and has a rotor which is moveably linked via a multi-grooved portion with a spline shaft of the plasticizing screw. Both electric motors are operated completely separated from one another.
A problem associated with electric motors is the increased costs, especially when high-performance motors are involved. Although small electric motors, which may serve as direct motors, are relatively inexpensive, the costs for high torque motors increase exponentially with increase in torque. The same is true for nut and spindle assemblies which rise dramatically in costs as the size of the assembly increases. Costs increase significantly in particular when large plasticizing units are involved. Still, the use of electromotive drives is desired because of their precise operation.
It would therefore be desirable and advantageous to provide an improved injection unit which obviates prior art shortcomings and which is simple in structure and cost-effective, while still being reliable in operation.